The distortion of audio signals for particular applications with musical instruments is known in the art. By way of example, the sound produced by the electric guitar is altered, in one aspect, dependent on the properties of the guitar amplifier. In one aspect, guitar amplifiers can be altered to emulate the sound produced by vacuum tube amplifiers, which were utilized when electric guitars were first developed.
Many musicians and audiophiles favor the sound produced by a vacuum tube amplifier. When a vacuum tube amplifier is overdriven, a warm distorted sound characteristic is produced with the presence of harmonics. Efforts have been made in the art to emulate that particular desired sound without using a vacuum tube amplifier.
By way of example, MOSFETs have been used to create tube amplifier distortion in the past. Craig Anderton, in “Electronic Projects for Musicians”, 1975, Project number 24, on page 170, discloses a “Tube Sound Fuzz” based on the CD4049 CMOS Inverter IC. This disclosure is based on an unbalanced circuit topology.
A balanced circuit topology is used for the distortion, clipping, or limiting of the signal. It is difficult to find devices that will enter into clipping at the same level and at the same rate on both polarities of their waveform. For transistors and MOSFETs, it is difficult to find devices that behave the same way for saturation and cutoff. For complementary MOSFETs it is difficult to find devices that have the complementary matching required to find a saturation characteristic that behaves the same for both P and N type devices. Because of this, it is difficult to generate a waveform that has symmetry. Waveform asymmetry causes beat notes at low frequencies when multiple musical notes are played together. The beat note from multiple tones causes modulation of the DC component and creates intermodulation at low frequencies. These low frequency intermods are very unmusical and, therefore, undesirable. A symmetric waveform, because it has no DC component, does not create this intermodulation at DC. Because the distortion circuitry is balanced, it does not create this intermod. The technology described herein suppresses this type of distortion. This type of clean distortion is what makes this disclosed technology an improvement over the present prior art.
The circuit topology disclosed herein lends itself well to the matching of components by the use of integrated circuits. Because of the circuit topology, matched components are needed in the two balanced paths. With matched components there may be asymmetry in a single path, but there will be an equal and matched asymmetry in the parallel path as well. These two paths, when subtracted will form the average of the two asymmetries. If they are matched, the output will be completely symmetric. Amplifiers known in the prior art require complementary matching of components which is considerably more difficult to obtain. Amplifiers known in the prior art require MOSFETs in which the P channel device's transfer characteristic has to be exactly equal and opposite to that of the N channel device. For the topology disclosed herein, the P channel device has to match the other P channel device, and the N channel device has to match the other N channel device, but the P channel device does not have to match the opposite of the N channel device. Because of the ease of implementation, this is considered an improvement to the prior art.
The circuit topology disclosed herein can also include the use of clamp diodes on the input of the distortion circuit to create cross-over distortion which is also a characteristic of an over-driven tube power amplifier. These diodes are inherent in CMOS inverters. When these inverters are built from discrete devices, the diodes can be switched into the circuit or can be switched out of the circuit to give a variation in tone. This disclosure provides a novel method for the emulation of tube cross-over distortion.
The circuit disclosed herein allows for power supply voltage variation on the distortion elements. This allows for variation in tone from a hard distortion to a smooth overdrive.
Related patents and published patent applications known in the art include the following: U.S. Pat. No. 4,987,381 issued to Butler on Jan. 22, 1991, discloses a tube sound solid state amplifier. U.S. Pat. No. 5,524,055 issued to Sondermeyer on Jun. 4, 1996, discloses a solid state circuit for emulating tube compression effect. U.S. Patent Application Publication No. 2008/0049950 filed by Poletti and published on Feb. 28, 2008, discloses a nonlinear processor for audio signals.
The foregoing patent information reflects the state of the art of which the inventor is aware and is tendered with a view toward discharging the inventor's acknowledged duty of candor in disclosing information that may be pertinent to the patentability of the technology described herein. It is respectfully stipulated, however, that the foregoing patent and other information do not teach or render obvious, singly or when considered in combination, the inventor's claimed invention.